A circuit board (sometimes referred to as a printed circuit board or a printed wiring board) is the basic building block for interconnecting electronic devices in a system. Electronic devices, usually integrated circuit (IC) devices, are mounted onto the circuit boards using a number of mounting mechanisms, by use of connectors or by directly mounting the devices on to a surface of the circuit board. A circuit board includes the wiring required to interconnect the devices electrically and to act as a primary support for the devices.
A circuit board typically includes multiple layers, which can range from two layers to fifty or more layers, as examples. Some layers are used for signal transmission, while other layers are used for power reference planes. A power reference plane is a plane that is connected to a reference voltage, such as ground, a three-volt voltage, a five-volt voltage, or some power supply other voltage. Some reference planes may even be coupled to more than one voltage.
As IC technology has steadily improved, the number of transistors that can be placed into each IC device has dramatically increased. This has led to a corresponding increase in the number of input/output (I/O) pins on such IC devices. The increase in I/O pins on IC devices means that more signals are routed on a circuit board, which in turn means a higher circuit wiring density on the circuit board. Also, as the number of layers increase to accommodate the increased number of signal lines, the number of vias in any given path also increases. A via is an electrical connection that is run through multiple layers of the circuit board to connect signal lines at different layers. Typically, the via is run perpendicularly to the main surface of the circuit board. In forming a via, some amount of dielectric material is removed by drilling, laser, light, or other methods. Next, an electrically conductive metal, usually copper, is flowed or plated into the void to provide the electrical connection between signal lines at different layers.
To avoid shorting problems, minimum clearances are defined between each via and the surrounding signal wires or reference planes. To maximize the density of wires that can be run through each layer of the circuit board, it is desirable for the spacing between the signal lines and a via to be the minimum possible while still avoiding short-circuit problems. Circuit board design and manufacturing methods require minimum clearance dimensions to be maintained equally on all board layers, regardless of whether the layer is a signal layer or a power reference layer.
Typically, a transmission line on a circuit board is formed between a signal trace (routed through a signal layer of the circuit board) and a reference plane (that is provided in a reference plane layer of the circuit board). A circuit board is designed to achieve transmission lines with a target characteristic impedance, which is usually 50 Ohms or 75 Ohms. The characteristic impedance of a transmission line is dependent on several factors: the inductance of the conductors that make up the transmission line, the dielectric medium, the distance to a reference plane, and the capacitance between the conductors.
Because vias are configured differently than signal traces, the characteristic impedance of each via is usually different than that of each transmission line. Usually, the impedance of a via is less than the characteristic impedance of signal transmission lines on the circuit board. As a result, if a signal path is routed through one or more vias between different layers of the circuit board, impedance discontinuities are introduced by the presence of the vias. For signals having low frequencies, the feature dimension of a via is usually much less than a wavelength of each signal. Therefore, such a low-frequency signal usually does not experience effects of impedance discontinuity introduced by the vias. However, as the frequency of the signal increases, the feature dimension of the via becomes a significant portion of, or is even larger than, a wavelength of the signal. The transmission line impedance discontinuity introduced by vias along a signal path can cause reflections, which degrades the signal and causes performance in a system to degrade.
The characteristic impedance of the via is usually lower than the desired transmission line impedance of 50 Ohms or 75 Ohms. Increasing the impedance of the via usually requires increasing the minimum clearance dimension (even on the signal layers carrying signal lines), which has an undesirable result of decreasing the area available for routing of transmission lines on signal layers. Decreased routing area is usually undesirable since the alternatives to combat this are to either increase the dimensions of the board, or to add signal layers to the board.